1. Field of the Invention
This invention relates to an apparatus for detecting an elevator speed, especially to one including a pulse generator which produces output pulses which correspond to the elevator speed and which are employed as an elevator speed signal.
2. Description of the Prior Art
In recent years, speed, landing operations, and the forces felt by passengers of an elevator system have been controlled through a feedback system which utilizes a detection of the elevator's speed.
In general, either a D.C. or A.C. tachometer has been employed for detecting the elevator speed. The tachometer is driven directly by a shaft of a traction machine or driven through a power transfer device such as driving belts. In either case, the shaft must be extended to drive the tachometer making it inconvenient to mount the traction machine within a limited space of a machine room and making it difficult to lift the traction machine for mounting. Further, in the case of the D.C. tachometer, necessary maintenance of a commutator and brushes is troublesome and, especially in the case where belts are employed, maintenance has proved particularly laborious. Moreover, a ripple component is usually included in the output of the tachometer due to vibration of the belt or the like.
To remedy these problems, A.C. tachometers have commonly been employed connected directly with the traction machine shaft. In this case, it is necessary to provide a rectifier which converts the output of the A.C. tachometer to D.C. Due to the forward voltage drop of the rectifier, no output signal from the rectifier appears when the speed of the traction machine is very low. Accordingly, an A.C. tachometer cannot be used for significantly low speed control.
To eliminate this defect, a speed detecting device is preferably used which includes a rotary disc formed in a predetermined shape and mounted on a motor shaft. An adjacently positioned pulse generator produces pulses proportional to the elevator speed in conjunction with rotation of the rotary disc. This enables the detection of the elevator speed with a high accuracy and makes unnecessary an increase in the required space for the traction machine. This type of speed detecting apparatus usually includes the pulse generator, a wave-forming circuit connected to the output of the pulse generator, and a smoothing circuit connected to the output of the wave-forming circuit for providing an analog speed signal. An example of this type of apparatus is shown in FIG. 1 and a waveform from the apparatus is shown in FIG. 2 to aid in an explanation of its operation.
In FIG. 1, a digital-to-analog converter 9 includes a monostable multivibrator 93 which generates output voltage pulses proportional to an input signal and a smoothing circuit 94 which produces an analog speed signal proportional to the input pulse rate. Both of these are connected to a D.C. power source 8. The multivibrator 93 includes a transistor 12 which is normally in the OFF state, another transistor 13 which is normally ON, a timing condenser 14 connected between the collector of the transistor 12 and the base of the transistor 13, a timing resistor 15 connected between the base of the transistor 13 and the positive terminal of a power source, a coupling resistor 16 connected between the base of the transistor 12 and the collector of the transistor 13, current limiting resistors 17 and 18 for limiting the collector currents, and an output terminal 31 connected to the collector of the transistor 13. Pulses proportional to the elevator speed produced by a pulse generating device (not shown) are supplied to the collector of the transistor 12 through an input terminal 29.
The smoothing circuit 94 includes a smoothing condenser 20 connected between a ground or earth terminal of of the power source and the output terminal 30, a smoothing resistor 21 connected between the power source and the collector of the transistor 25, another smoothing resistor 22 connected between the collector of the transistor 25 and the output terminal 30 having the same resistance value as the smoothing resistor 21, a diode 23 connected in parallel with the smoothing resistor 22 to permit only a charging current to flow into the smoothing condenser 20, a transistor 24 with its collector connected to the power source through a current limiting resistor 26 and its base connected to the terminal 31 through a resistor 27, and a resistor 28 connected between the collector of the transistor 24 and the base of the transistor 25. The charging and discharging circuits for the condenser 20 have the same time constant. The emitters of each of transistor 12, 13, 24 and 25 are connected to ground.
In the prior art construction shown in FIG. 1, while the elevator cage and a counterweight are moved vertically up and down by the traction motor through a sheave and main rope, the pulse generating device produces pulses at a rate proportional to the speed of the elevator cage. The multivibrator 93 generates output pulses of a constant pulse width at output terminal 31 whenever the output pulses of the pulse generating device are supplied to the input terminal 29. The frequency of the output pulses generated by the multivibrator 93 is the same as that of the input pulses while the width of each output pulse is independent of the input pulse frequency. The output pulses from the multivibrator 93 are smoothed by the smoothing circuit 94. Specifically in regard to the smoothing operation, when the output at terminal 31 reaches a high level, the transistor 25 turns off and the smoothing condenser 20 is charged from the power source 8 of output voltage Vc at a rate set by the time constant determined by the values of the capacitance of the smoothing condenser 20 and the resistance of the smoothing resistor 21. When the output at terminal 31 drops to a low level, the transistor 25 turns on and the smoothing condenser 20 is discharged at a rate set by the time constant determined by the capacitance of the smoothing condenser 20 and the resistance of the smoothing resistor 22. Since the resistances of the smoothing resistor 21 and the smoothing resistor 22 are equal, the charging time constant is equal to the discharging time constant. Therefore, the voltage across the smoothing condenser 20, that is, the mean output voltage at the output terminal 30, is proportional to the frequency of the input pulses on the input terminal 29.
In the conventional construction, the monostable multivibrator was constructed as described above, so that the analog output on line 9a of the smoothing circuit 94 changes undesirably according to voltage changes of the power source 8. This will be apparent from the description which follows with reference to FIG. 2. In the conventional monostable multivibrator, it is known that the output pulse width does not change in dependence on the power source 8 voltage. This is seen in the waveform of FIG. 2 which represents the electric potential at the base of the transistor 13. However, the output pulse height of the multivibrator 93 changes according to voltage changes of the power source 8 such as from voltage Vc.sub.1 to voltage Vc.sub.2 in FIG. 2. Voltage changes in the power supply also cause voltage fluctuations at the output terminal 30. Therefore, the analog output, that is the analog speed signal on line 9a at the output terminal 30' changes according to the power supply voltage change. This means that a speed signal proportional to the elevator speed cannot be properly produced if the voltage of the power source 8 is subject to fluctuations.
Moreover, the smoothing circuit 94 cannot completely smooth the output of the multivibrator 93. Accordingly, the analog speed signal on line 9a has a ripple component and this ripple component gives rise to inaccurate control of the elevator speed especially at low speeds. In order to reduce the ripple component percentage of the analog speed signal, the smoothing circuit may be provided with a longer time constant. But a long time constant is undesirable for accurate and responsive control of the elevator speed.